Present methods commercially employed for the desulfurization of chars, such as those made from coal, only remove a portion of the sulfur present therein. This is inadequate for purposes of complying with air purity standards which limit the amount of sulfur dioxide produced by the combustion of coals, chars, and other carbonaceous materials. The need for a relatively sulfur-free char, and especially a coal char, has therefore become very important, especially in view of the abundant supplies of coal and the dwindling supplies of oil and natural gas.
One desulfurization method used on coal is to treat the coal with hydrogen gas to react with the sulfur in the coal to form hydrogen sulfide gas. This method may work well with coal but not as well with chars made from coal because the sulfur appears to be more resistant to reaction with hydrogen after being converted into char.
A further problem in treating coal or char with hydrogen gas is that trace amounts of hydrogen sulfide in the hydrogen gas will inhibit desulfurization or back-react with the coal or char to increase its sulfur content. Hydrogen gas used in coal or char desulfurization methods is usually recycled and will contain small amounts of hydrogen sulfide and other sulfur-bearing gases produced during desulfurization. Attempts may be made to remove the sulfur-bearing gases from the hydrogen before recycling, but such attempts are not too successful unless the gases are subjected to more elaborate and expensive separation techniques. As a result, the hydrogen used for desulfurization purposes may contain small amounts of sulfur-bearing gases and when it does, desulfurization is inhibited or a back reaction may occur to increase the sulfur content of the coal or char. Consequently, in methods that do employ recycled hydrogen gas, it is important that all traces of sulfur-bearing gases be removed therefrom, which, as a practical matter is difficult to do.